One of the major problems of peripheral vascular control has to do with the mechanisms determining the distribution of blood flow between parallel circuits located within a single tissue type. The isolated dog hindpaw has two major parallel vascular circuits, 1) capillary or nutritional circuit and 2) arteriovenous anastomoses (AVA) or non-nutritional circuit. The sympathetic nerve fibers controlling the resistance and capacitance vessels in this area are grouped into three trunks each apparently affecting different series coupled macro and microvascular segments as well as the two major parallel circuits. Therefore a unique model is presented that allows definition of the precise characteristics of the responses at both the macro and microvascular level to direct, CNS and reflex stimulations in the same preparation. These will include direct electrical stimulations of the nerves, electrical and thermal CNS stimulations, reflex thermal stimulations and baroreceptor and chemoreceptor reflex activation. It is expected that the control mechanisms may well be dependent upon the necessity for heat loss or retention as well as blood pressure control. Blood flow, large vessel pressures, small artery and vein pressures, microvessel diameter changes by videomicroscopy, total tissue volume changes (plethysmography), vascular volume changes (indicator dilution), and capillary availability (PS and CFC) will all be measured in the same preparation. The study of this vascular area should provide basic information since the direct, reflex and central effects on peripheral vascular areas other than cutaneous are also not understood, e.g. the intestine where similar parallel circuits occur. In the hindpaw neural efferent outflows can be relatively specifically identified, and the redistribution of flow and volume between parallel circuits, and the contributions of the series coupled macro and microvessels to these shifts can be effectively explored in this preparation.